With the development of semiconductor technology, the efficiency of an LED has been significantly improved. Since the LED has a larger lifetime and smaller energy consumption than existing lighting devices such as incandescent lamps or fluorescent lamps, the LED is economical and environment-friendly. Due to such advantages, much attention has been paid to the LED as a light source to replace a back light of a traffic light or flat display (for example, LCD).
In general, when the LED is used as a lighting device, a plurality of LED modules each having a plurality of LEDs mounted therein are disposed in a specific arrangement and then installed on the ceiling or wall surface of a space so as to be used. Since the LEDs are driven by direct current at a proper voltage, each of the LED modules includes an AC/DC converter. The AC/DC converter includes a trans-coil for lowering an AC voltage. Since such a trans-coil occupies a considerably large space in the LED module, the size of products to which the trans-coil is applied is inevitably increased.
In order to solve such a problem, a power supply device called a switching mode power supply (SMPS) has been recently used. However, a recent LED lighting device includes a plurality of LEDs connected in series without the SMPS, and is driven in such a manner that each of the LEDs lowers the voltage with respect to power.
However, the conventional LED lighting device could not properly control the drive of the LEDs in response to driving power which is varied in accordance with time. Accordingly, the efficiency and power factor may be reduced, and THD may be increased. Furthermore, when fluctuation occurs in input driving power, brightness may be varied.